Confused about crawl space ventilation.

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  #1  
Old 03-24-17, 09:10 AM
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Confused about crawl space ventilation.

12 standard foundation vents..... about 2000 sq.ft. and 4000 cubic feet. 6 mil plastic on soil. We are in southern OR. HVAC ducts in crawl space and R21 fiberglass batts between joists.

I have a $20 temp/humidity sensor in the crawl space. It now reads 55 degrees and 76 percent humidity. Weather station across the road from us says: 45 degrees outside temp and 97 percent humidity. It is now raining.

I can see that temperature can be different...but why humidity difference ? Is my sensor off that much? (AccuRite 02097M).....Or does the HVAC ducting affect the humidity. Ducts are well sealed (I think) and insulated.

Should the humidity be the same %....outside and under the house ?

Been looking at Tjernlund....SmartVent and Atmox.

Any comments appreciated
 
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Old 03-24-17, 10:12 AM
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The humidity is not the same because the temperature and dewpoint are not exactly the same. Take 45F air at 97% and the dewpoint is about 44F.

Same air inside is warmer... 55F. If the dewpoint is still "close to" 44F the humidity will be around 76%.

If it confuses you, look up the definition of relative humidity.
 
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Old 03-24-17, 11:35 AM
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Thanks, XSleeper. I have bookmarked a dew point calculator at [url]www.dpcalc.org... still am confused.

While considering ventilation, it seems that if I can send less humid air into the crawl space and/or exhaust more humid air......that is the way to go. ??

Now have a desk oscillating fan in the crawl space running 0800 to 2000. Does this help ?
 

Last edited by YaddaYadda; 03-24-17 at 11:51 AM.
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Old 03-24-17, 11:56 AM
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You are worrying about it too much. Your humidity in the crawl space will probably always be high. In winter months should be lower due to dry cold air outside. But if you introduce power ventilation, in summer months the humidity will likely soar because whatever you exhaust is going to be replaced from outside. The crawl space will be cooler than it is outside and as a result the RH will be higher.

If you introduce humid 85F air in the summer it will likely cause the RH in your cooler crawl space to get into the 80s.

The only real way to bring the RH down and keep it more constant is to encapsulate the crawl space, close vents, and condition the space with heating, cooling, and either returned air or positive pressure exhaust.
 
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Old 03-24-17, 12:43 PM
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OK, I'll dial back the coffee for a while. I summarized water intrusion into my crawl space, earlier today.....now I am looking into keeping the crawl space dry.

What do you think of my oscillating fan.....?

Thanks
 
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Old 03-24-17, 02:31 PM
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A fan is good if the floor or walls are wet.
 
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Old 03-24-17, 11:35 PM
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I think you are being confused by your use of the term humidity without its modifier, relative. It is ALWAYS relative humidity, relative to the air temperature. As the temperature changes the relative humidity is also going to change because it is a measurement of the percentage of moisture in the air compared to the maximum amount of moisture that air at that temperature can hold before it starts to condense. That is why relative humidity is always expressed as a percentage.

Cold air will hold a lesser amount of water vapor than will warmer air. For any given sample of air as the temperature rises the relative humidity will drop although the given sample of air will still have the same amount of water vapor.
 
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Old 03-25-17, 06:05 AM
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To say it in another way, temperature and humidity act as a seesaw with the dew point as the pivot. Temp goes up humidity goes down and vice versa.

In a humid climate ventilating the crawlspace is not going to do well at reducing the humidity down there. Encapsulation is the best way along with sharing the conditioned air from the house above, ac and heat.
Five Ways to Deal with Crawl Space Air | GreenBuildingAdvisor.com

Bud
 
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Old 03-25-17, 01:38 PM
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I agree with Bud that an encapsulated crawlspace is best but the cost is also high. I've seen estimates of eight to ten thousand dollars to encapsulate an existing crawlspace.

The biggest thing is to have the crawlspace a non-condensing environment. This means keeping the temperature higher than the dew point, the temperature where the moisture condenses out of the atmosphere. I live in the land of perpetual rain, have a ventilated crawlspace and no problems. I also have a pair of "muffin" fans mounted in one of the ventilation openings and connected to a timer. I run these fans from midnight to four AM and again from noon to four PM. The cost of operation is negligible.
 
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Old 03-25-17, 06:02 PM
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Hi Joel,
I'm guessing (hoping) your estimated cost to encapsulate that crawlspace is on the high side. Floor is already covered with plastic and wall height is about 2'. With a perimeter 180' for a 2,000 ft² foundation (40' x 50') that would be 360 ft² of wall area. Even upgrading to 2" Thermax to meet both code level insulation requirements and provide the thermal covering, costs would only be about $2 per ft² (been awhile since I priced it). But that is well under $1,000, DIY. Some added cost to detail the rim area and then supply the needed air exchange.

Just back of hand thinking.

Bud
 
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Old 03-25-17, 09:45 PM
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Bud, I'm only going by what I have read on other posts where encapsulation has been considered and the prices I have seen on websites that offer the products. Floor/ground coverings are generally multi-ply and about 30 mils in thickness vs. the 6 mil single ply the OP has. Specialty tapes for the seams as well as fasteners for the sidewalls and boots for the posts/pillars all add up.
 
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Old 04-09-17, 11:17 AM
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Thanks for all your help. A while back I had a big, national, firm here to see about a simple fix of tattered/torn plastic, look for water entry points, etc. The recommendation was total encapsulation at a special price for me, today, of $22,000.

So, after reading and asking around....I am working on this: When outdoor dew point is near or equal to crawlspace temperature.....that is when a de-humidifier is needed.

If outdoor dew point is below crawl space temperature....all is well and no action needed.

How am I doing ?
 
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Old 04-09-17, 01:49 PM
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That is on the right track. The only time the humidity in your crawlspace will have "any chance" of going lower will be the days when the exterior dewpoint is much lower than the crawlspace ambient temperature. But due to the constant low ground temperature, there will be many days where the dewpoint outside will be above the dewpoint. You have to be cautious about power venting because the low temperature in the crawlspace will become more humid if you introduce lots of fresh humid air. (When dewpoint outside is above the temperature inside).

Generally no action is needed in vented crawlspace when the rate of air exchange is limited / low. A dehumidifier is never a bad thing.
 
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Old 04-09-17, 02:40 PM
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Thank you XSleeper...much appreciated.
 
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